The present invention relates to an electronic control system for controlling a secondary pressure and a transmission ratio by a solenoid operated valve of a continuously variable belt-drive transmission, and more particularly to the system for correcting a dither applied to a control current for the solenoid operated valve.
A hydraulic circuit of the continuously variable transmission (CVT) for a motor vehicle has the solenoid operated control valve to control a primary pressure and the secondary pressure in the circuit. The inventor of the present invention has disclosed an electronic control system in Japanese Patent Application 2-8718 where a solenoid is provided in the control valve, which is linearly operated by an analog exciting current In order to reduce hysteresis movement of the valve, the dither which cyclically oscillates at a predetermined amplitude is added to a basic exciting current to produce a dither exciting current. The exciting current is applied to the solenoid.
The dither exciting current oscillates in a positive and a negative directions at an amplitude with respect to the exciting current, so that the mean exciting current is equal to the basic exciting current. Accordingly, the control valve constantly vibrates with pulsations of the dither exciting current, thereby reducing the hysteresis movement of the valve.
On the other hand, rising characteristics of the dither current largely changes with an impedance of the coil (solenoid) dependent on temperature of oil provided in the continuously variable transmission and with variation of the output voltage for the control unit dependent on balance of supply and consumption of power in the vehicle. Consequently, the mean exciting current largely changes, thereby deviating from the basic exciting current. Hence the solenoid operated control valve cannot be accurately controlled.